In an earlier post, we described the El Zacatón sinkhole, the deepest water-filled sinkhole known at present anywhere on the planet. Such a large sinkhole begs some important questions:

How did such a large sinkhole form?

Most major sinkholes form as a result of the collapse of the ceilings of underground cavities which have been formed by the gradual dissolution of limestone due to percolating acidic rainwater. However, according to Marcus Gary, of the Jackson School of Geosciences at the University of Texas at Austin, who has studied this area since the 1990s, the Zacatón pit is not a conventional sinkhole. He believes that this pit began to form in the Pleistocene period as a result of underground volcanic activity. Volcanism increased the acidity of water deep underground which then gradually ate away at the surrounding limestone in a process known as “hypogenic karstification“. As the underground caverns grew larger, the overlying rock would periodically collapse into them, eventually leaving giant pits extending to the surface above.

Is the sinkhole continuing to get deeper?

Equally interestingly, some of the sinkholes appear to be closing over. All the major sinkholes in Tamaulipas contain lakes and in several cases, they appear to be crusting over with travertine, a form of calcium carbonate which, in the right conditions, can be precipitated out of calcium bicarbonate-rich water. According to Marc Airhart, another researcher at the Jackson School of Geosciences, the process is probably an excruciatingly slow one, but at least one sinkhole (Poza Seca) has closed up entirely, sealing off an underwater lake. This travertine skin may also explain why El Zacatón has floating islands. It seems likely that pioneer species may have colonized small rafts of travertine, beginning the series of ecological processes that resulted in the grassy islands that can be seen there today.